Polyphase synchronous rectifiers are used throughout the power electronic industry to convert an input AC voltage to an output DC voltage. Three-phase converter circuits, for example, are used to convert voltage in such applications as motor drives and Uninterruptible Power Supply (UPS). A conventional three-phase boost rectifier circuit typically includes AC to DC circuitry of active and passive components such as an input inductance, power switches and an energy storage device (commonly a capacitor). Often, the AC to DC circuitry is connected to a DC to DC converter for additional conversion and to a controller for controlling the operation of the power switches. To provide a desired output DC voltage and unity power factor, the controller employs different control schemes based on the input AC voltage and feedback from the output DC voltage and AC input current. One control scheme presently employed in three-phase boost rectifier circuits is a vector control scheme.
Embodied within a Digital Signal Processor (DSP), the vector control scheme can be implemented with great flexibility to achieve a high dynamic performance. The vector control scheme includes a voltage loop to regulate the output DC voltage and current loops to control input AC currents. Additionally, the vector control scheme includes a zero crossing detector that detects a phase on an input AC supply line and produces a zero-crossing signal for a 3-2 transform circuit and a 2-3 transform circuit within the current control loops. The transform circuits provide an interface between a three-phase stationary frame (real world) and a two-phase rotational reference frame. Transforming to the two-phase rotational frame allows manipulation of vectors representing the input currents, input voltages and error currents to provide control for the power switches to obtain the desired AC currents and output DC voltage.
The rotational reference frame, such as a d-q frame with perpendicular d and q axes, is adopted and locked to an input AC voltage vector V to provide a reference for the vector control scheme. To obtain a unity power factor, a reference vector I* will be in phase with the voltage vector V in the d-axis. The current controllers generate required voltages Vd and Vq from error current components in the d and q axes that pass therethrough. The 2-3 transform circuit will receive the required voltages Vd and Vq and generate a terminal voltage drive vector Vt that is applied to the AC to DC circuit through Pulse Width Modulation (PWM) to control operation of the power switches.
Generally, conventional vector control schemes as discussed above are sufficient when the input AC voltage is symmetric. Unfortunately, when an input AC voltage source is non-ideal (asymmetric or unbalanced), current distortion occurs with conventional vector control schemes.
Accordingly, what is needed in the art is an improved apparatus or method to control poly-phase rectification. More specifically, a poly-phase rectifier controller is needed that reduces current distortion even with an asymmetric input AC voltage.